How is the 'Relevance Index' metric calculated?

And I can see how Query Count and Click Rank influence the Relevance Index, but I want to understand the detail under the calculation better.

For example: We receive a suggestion for the Thesaurus to Expand Any for a set of terms that are related. At first glance, my gut feeling is that expanding the terms to each other would bloat the search results for some of the terms, reducing relevance. Is there a way I can model making this Expand Any suggestion to see how it would affect the Relevance Index score for each keyword in the set of terms? I'm essentially looking for a Before and After picture without having to actually implement the Expand Any suggestion - hence my interest in how Relevance is calculated.